Cooling of pellets from PET solid stating reactor with water

ABSTRACT

Energy savings are realized during the commercial production of polyethylene terephthalate by partially cooling polyethylene terephthalate pellets exiting a solid stating reactor by contact with water, and using the residual heat stored in the pellets to vaporize associated water to form dry pellets.

This application is a Continuation of U.S. application Ser. No.10/696,880 filed Dec. 18, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the commercial manufacture ofpolyester polymers, in particular, polyethylene terephthalate (“PET”)polymers.

2. Background Art

PET has numerous uses, principle among which are for films, fibers, andfood containers. Despite the stringent matrix of properties required forsuch uses, particularly for food packaging, PET has become a commoditypolymer. Commercial production of PET is energy intensive, and thereforeeven relatively small improvements in energy consumption are ofconsiderable commercial value.

The production of PET (inclusive of copolymers) begins with anesterification step where the dicarboxylic acid component, predominantlyterephthalic acid, is slurried in ethylene glycol and heated to producea mixture of oligomers of a low degree of polymerization. This“esterification” step may be followed by a further “oligomerization” or“prepolymer” step, where a higher degree of polymerization is obtained.The product still has a very low molecular weight at this stage.

The previously described steps are then followed by a polycondensation.The polycondensation is catalyzed by metal compounds such as Sb, Ti, Ge,Sn, etc. Polycondensation occurs at relatively high temperature,generally in the range of 280-300° C., under vacuum, water and ethyleneglycol produced by the condensation being removed. The polymer at theend of polycondensation has an inherent viscosity generally in the rangeof 0.4 to 0.65, corresponding to a molecular weight too low for manyapplications.

Commercial production of PET and other polyesters as well has required asubsequent post-polymerization in the solid state, termed “solidstating.” In this stage of the process, polyester pellets are heated ininert gas, preferably nitrogen, in a solid state polymerization reactor,often termed a “solid stating reactor” or “solid stater”, attemperatures below the melt temperature, i.e. from 210-220° C. in thecase of PET. Solid stating is complicated by the fact that most PETpolymers and other polyesters as well, following extrusion from the meltand pelletizing, are substantially amorphous. In order to prevent thepellets from sintering and agglomerating in the solid stater, thepellets are first crystallized over a period of 30 to 90 minutes at alower temperature, e.g. 160-190° C., typically in a flow of inert gas.It should be noted that “solid stating” herein refers to the solid statepolycondensation per se, and not to the combined processes ofcrystallization and solid state polycondensation.

Following polycondensation in the solid state, it has been the practiceto cool the pellets in a stream of cool air or nitrogen gas, which isthen cooled and recycled. Considerable quantities of gas are required,as well as circulation pumps of large capacity. Moreover, the equipmentrequired for cooling is large, and thus capital intensive. Use of waterfor cooling is not known, most likely because it had been thought thatwater associated with water-cooled pellets required complete removal, asotherwise it may cause polymer hydrolysis during processing steps suchas extrusion and injection molding. For these reasons, PET pellets arethoroughly dried before use.

In U.S. published application 2003/0039594 A1, a method is disclosed forcooling hot polymer pellets from a solid stating reactor where aconventional fluidized bed cooler is used, but augmented by water sprayinto the cooler proximate the hot pellet inlet. A first section ofcooler is isolated from a further section, the first section operatingminimally at 230° F. to avoid overwetting of pellets. The object of the'594 publication is to utilize the heat of vaporization of water to aidin pellet cooling, while also reducing the flow of gas to the cooler.However, by spraying water over the bed in the heated chamber,considerable water is vaporized by contact with hot gas rather than hotpellets, and when a closed gas recirculation system is employed, adehumidifier must be added to the gas recirculation line. Not only doesthe process of the '594 publication involve only a modest improvement inenergy usage in the pellet cooling process, it moreover requiresmonitoring and adjustment of additional parameters in the fluidized bedcooling unit.

It would be desirable to provide a process for cooling pellets whichdoes not require a large volume air stream, and yet which providespellets which are suitable for later processing by conventional moldingtechnology such as injection molding.

SUMMARY OF THE INVENTION

The present invention is directed to a process for cooling hot pelletsexiting a solid stating reactor wherein the hot pellets are contactedwith water in the liquid phase to cool the pellets to a temperaturepreferably within the range of 50° C. to 120° C., and optionally usingthe residual heat contained in the pellets to evaporate water associatedwith the pellets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the prior art method of cooling hot PET pelletsexiting a solid stating reactor.

FIG. 2 illustrates one embodiment of a process for cooling hot PETpellets in accordance with the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention pertains to any polyester which is solid state polymerizedat high temperatures. The most common of such polyesters is PET, and theremainder of the specification will be addressed to this polymer asillustrative of the process which can be employed with polyestersgenerally.

The PET polymers are conventional, and are polymers prepared fromterephthalic acid and ethylene glycol. While dimethylterephthalate mayin principle be used as well as terephthalic acid, use of the latter ispreferred. In addition, the PET polymers may contain up to 20 molpercent, preferably up to 10 mol percent, and more preferably no morethan 5 mol percent of dicarboxylic acids other than terephthalic acid,and the same mol percentages of glycols (diols) other than ethyleneglycol.

Examples of other suitable dicarboxylic acids which may be used withterephthalic acid are isophthalic acid, phthalic acid, naphthalenedicarboxylic acids, cyclohexane dicarboxylic acids, aliphaticdicarboxylic acids, and the like. This list is illustrative, and notlimiting. In some cases, the presence of minor amounts of tri- ortetracarboxylic acids may be useful for generating branched or partiallycrosslinked polyesters. Isophthalic acid and naphthalene dicarboxylicacids are the preferred dicarboxylic acid when mixtures of acids areemployed.

Examples of diols other than ethylene glycol which may be employedinclude, but are not limited to, 1,2-propane diol (propylene glycol),1,3-propane diol (trimethylene glycol), diethylene glycol, triethyleneglycol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol,cyclohexanediol, neopentylglycol, and cyclohexanedimethanol. Preferredglycols other than ethylene glycol include diethylene glycol, and mostpreferredly, cyclohexanedimethanol (“CHDM”), the latter generally usedas a mixture of isomers. In addition, polyols such as pentaerythritol,glycerine, and trimethylolpropane may be used in most minor quantitieswhen branched or partially crosslinked polyesters are desired. Mostpreferably, only difunctional carboxylic acids and difunctionalhydroxyl-functional compounds (glycols) are employed.

The esterification, oligomerization, and other process steps up to andincluding the solid stating reactor are conventional. Polycondensationin the solid state generally occurs in a reactor termed a “solid statingreactor,” at temperatures within the range of 200° C. to a temperaturejust below the melt temperature of the polymer, i.e. 2-10° C. below themelt temperature of the polymer, and thus pellets exiting the solidstating reactor typically have a temperature in excess of 180° C.Pellets of polyesters with higher melting points may be polycondensed atcorrespondingly higher temperatures.

In the inventive process, the “hot” pellets are contacted with aquantity of liquid water such that the temperature of the pellets islowered, preferably to within the range of about 50° C. to about 120°C., more preferably 50° C. to 90° C. Following cooling to the desiredtemperature, the pellets are separated from water by conventional means,such as a screen, perforated plate, centrifugal separator, or the like.All water need not be removed, and the pellets may appear wet followingwater removal. At this stage, the pellets desirably contain less than60% by weight water, preferably less than 25% by weight. The pellets maythen be dried, if desired, in a conventional pellet dryer, to a yetlower water content.

The pellets, regardless of whether the optional dryer is used, aresuitable for all molding processes for which PET pellets areconventionally used. The thorough drying at the customer site, i.e.where molding is to take place, will remove the last vestiges of water,and hence full drying during the production phase can be dispensed with.When an optional dryer is used, absolute dryness is not necessary, andthe air flow (or nitrogen) through the dryer at this stage is minimal,and far less than that required to cool the pellets in the absence ofwater cooling.

In preferred embodiments, the temperature of the pellets may be suchthat the pellets, together with any remaining water associated withthem, can be dried without the addition of more than a minor amount ofheat, preferably without the addition of any heat. It is preferable thatthe temperature of the pellets be slightly in excess of the temperaturerequired for volatilization of water, in which case the product pelletswill be substantially dry prior to packaging, even in the case whereprocess excursions vary the mass of pellets relative to the mass ofassociated water at any point in time. The pellet temperature may behigher or lower than the range of 50° C. to 120° C. in some cases,particularly on the high end of this preferred range. For example, atemperature of greater than 120° C. may be desirable if an unusuallylarge amount of water is associated with the pellets, and it is desiredto eliminate this water substantially completely.

When this “self-drying” is desired, the cooled pellet target temperaturecan easily be calculated approximately beforehand, taking into accountthe heat capacity of the particular polyester, generally about 0.44cal/g.° C., the amount of water which will remain associated with thepellets, and the desired prepackaging temperature of the pellets, i.e.the temperature of the dry pellets exiting the dryer. The amount of heatrequired to remove water is approximately equal to its heat ofvaporization, while the heat released by the cooling pellet is C_(p)T,where C_(p) is the heat capacity of the polyester and T is the change intemperature of the hot, wet polyester pellets and the dry, “cool”polyester pellets. A more exact calculation can easily be made.

When a dryer is used, in order to operate efficiently, the bulk of thecooling water should be removed by mechanical means, i.e. bycentrifugation, use of filter screens, etc., as described previously.These techniques are well known. Water may also be removed by mechanicalmeans in the dryer per se.

In the case where only minimal water remains associated with thepellets, drying may be accomplished simply by evaporation on a travelingbelt or other means. Generally, however, the pellets are introduced intoa non-heated drier, or a drier heated only by hot gas and/or water vaporwhich is derived from initial contact of the hot pellets with coolingwater. Thus, it is preferable that no external heat, e.g. heat notsupplied by the PET production process itself, be supplied.

The type of dryer used is not critical. Both agitated dryers as well asfluidized bed dryers may be used, and are commercially available. Dryingpreferably takes place in a stream of air or inert gas, e.g. nitrogen,or under reduced pressure. Water vapor removed from the dryer may becondensed, if desired, and mixed with the pellet cooling water, which ispreferably reused. By “dry pellets” is meant pellets which appear dry tothe touch, e.g. pellets which contain substantially no surface water.Some water may be absorbed within the pellets, however this amount isgenerally small.

Should the cooling water be hotter than desired coming from the dryer,water separator, etc., it may be cooled by a refrigeration unit or by aheat exchanger such as a radiator (for air cooling) or heat exchanger(for liquid cooling). In the case of heat exchangers employing liquidcoolants, plant water or river water may be used as the cooling medium.

FIG. 1 illustrates the prior art technique of pellet cooling. Hotpellets enter the cooler 1. Nitrogen gas is introduced into the coolerby line 3. While in the cooler 1, the gas is heated as the pellets cool,and hot gas exits the cooler through line 4, where it is pumped byblower 6 through chiller 7 and recycled back to the cooler. Cool pelletsexit the cooler through outlet 8 for packaging, loading into bins,railroad cars, etc. The exit temperature of the pellets is preferablyabout 50° C. or less. Due to the low heat capacity of nitrogen, aconsiderable flow of gas is necessary, and due to the size of the coolerand capacity of the pumps, considerable capital investment is required.

The subject invention may be illustrated by FIG. 2. Pellets in the solidstating reactor 10 exit the reactor through line 11, at a temperatureof, for example, 210° C. to 220° C., in the case of PET. Water flowsthrough line 12, into which hot pellets from line 11 are directed. Amixture of water and pellets flows through line 13 to an optional, butpreferable water separator 14, which may be, for example, a foraminousscreen, centrifugal separator, etc. The pellets then enter dryer 16,assisted by an optional flow of air 15. After drying, the pellets aredirected through line 17 for shipment or packaging.

The amount of water input to line 12 is, in general, considerablygreater than the amount of pellets, on a volume/volume basis, so that aflowable slurry of particles in water may be formed. The amount of waterrelative to pellets on a weight basis is such that by the time thepellets reach the dryer 16, they are below 140° C., preferably in therange of 50-120° C., and most preferably in the range of 50-90° C.

Water removal preferably takes place at 14 by conventional water removalmethods currently practiced, for example that practiced in removingwater from wet pellets from the pelletizer prior to entry into thecrystallizer of the conventional PET process. Pelletization isordinarily conducted under water, and thus the pellets are associatedwith a relatively large amount of water at this stage of the process.Foraminous surfaces, i.e. screens, perforated plates, etc., aregenerally used for this purpose, and may be vibrated screens, movingscreens, and the like. Pellets may be urged to continue past the screenby a flow of air, by mechanical means, or where the screens are tiltedfrom the vertical, by gravity.

The dryer 16 may be a commercial drying unit, as is now commonlyemployed prior to the crystallizer in the conventional PET process. Suchdryers are well known, and are available from numerous sources such asBepek, Gala, and Reiter. In such dryers, drying is ordinarilyaccomplished through the use of hot air. However, in the present case,the air is preferably not heated, as the pellets themselves are stillrelatively warm. Due to water contained on or with the pellets, thetemperature of the pellets will rapidly fall as the water evaporates,and the temperature of the relatively dry pellets exiting the driershould preferably by about 60° C. or less, preferably 50° C. or less.

The drier may also be of the centrifugal type, where a considerableportion of water associated with the pellets is removed by centrifugalforce, while a further portion evaporates. Such a centrifugal dryer canperform both the functions of initial water removal which mightotherwise by performed by a foraminous water removal device, and furtherdrying of the pellets, optionally aided by a gas stream.

The “dry” pellets which are packaged or shipped may appear truly dry, ormay be “damp” pellets, e.g. containing up to 10 weight percent water.

The present invention has numerous advantages over the prior art. First,due to contact with liquid water in relatively large quantity, thepellets are rapidly cooled, and thus the amount of gas recirculated willbe limited to that involved in the drying unit, when the latter is used.In the case of mechanical dryers such as centrifugal dryers, very littleif any gas will need to be recirculated.

Second, due to the fact that the pellets are rapidly cooled to arelatively low temperature, air may be used for subsequent operationsrather than nitrogen or other inert gas. Thus, the expense of employinglarge quantities of nitrogen is eliminated.

Entry of pellets into the cooling water may take place through use of anair stream, by gravity flow, or by spraying with jets or sprays ofwater. If the latter are used, they may take the place of a flow ofwater into line 12. However, it should be noted that the total amount ofwater employed at this stage should be such that the pellets are stillrapidly cooled to preferably 50-120° C., and the pellets are actuallywet with water at this point. Use of an amount of water whichimmediately vaporizes while leaving the pellets at a temperature higherthan 120° C. is not contemplated by the invention, as then cooling airwill again be required, and the full benefits of the invention will notbe obtained.

Water separated in the dryer or prior thereto is preferablyrecirculated, and will likely require cooling. The amounts of coolingwater required to be circulated is minimal, as opposed to the muchhigher volume of air or nitrogen which would otherwise be required.Moreover, chilling of the recovered water with river or plant water isentirely feasible due to the relatively small volume involved, andcooling efficiency is high, whereas with large volumes of gas, heatexchange is less practical and somewhat inefficient. Overall, theprocesses described herein offer a considerable savings in capital costsand operating costs.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A process for cooling polyethylene terephthalate pellets exiting a polycondensation solid stating reactor in a polyethylene terephthalate production process, comprising contacting pellets exiting a solid stating reactor with liquid water in an amount sufficient to lower the temperature of said pellets, removing liquid water from said pellets, and recovering cooled pellets containing about less than 25 weight percent or less of water.
 2. The process of claim 1, wherein said step of contacting is effected by at least one spray of water which contacts said pellets.
 3. The process of claim 1 wherein said step of contacting comprises directing pellets exiting said solid stating reactor into a moving stream of water.
 4. The process of claim 1, wherein following said lowering of the temperature of said pellets, said pellets are introduced into a mechanical dryer.
 5. The process of claim 4, wherein said dryer is a paddle dryer or a fluidized bed dryer.
 6. The process of claim 4, wherein pellets are separated from at least a portion of water associated with said pellets by mechanical means before entry into said dryer or within said dryer.
 7. The process of claim 4, wherein drying is effected without the addition of external heat.
 8. The process of claim 4, wherein said dryer is heated by process heat derived from another portion of said PET production process.
 9. The process of claim 1, wherein water used in the process is recovered and recirculated to the process.
 10. The process of claim 9, wherein prior to contacting pellets in water, water being recirculated to the process is chilled.
 11. The process of claim 10, wherein water being recirculated is chilled by means of a heat exchanger.
 12. The process of claim 1, further comprising removing liquid water from wet pellets, said wet pellets having a first temperature of from about 50° C. to about 120° C. to provide moist pellets having a first water content of less than 60% by weight, and volatizing water from said moist pellets due to heat retained by said pellets, and recovering pellets having a second temperature lower then said first temperature and a water content lower then said first water content.
 13. The process of claim 12, wherein said step of volatizing water takes place in a mechanical dryer in a flow of gas.
 14. The process of claim 13 wherein said gas is not heated prior to entry into said dryer.
 15. The process of claim 12, wherein the water content of pellets following said step of recovering is less then 2% by weight.
 16. The process of claim 1, wherein exiting said solid stating reactor are contacted with liquid water in an amount sufficient to lower the temperature of said pellets to a temperature within the range of about 50° C. and 120° C.
 17. The process of claim 4, wherein the temperature of the pellets exiting said dryer are 60° C. or less.
 18. The process of claim 1, wherein residual heat contained in the pellets exiting a solid stating reactor is used to evaporate water contacting the pellets exiting the solid stating reactor. 